1. Technical Field of the Invention
The present invention relates to a colored image forming apparatus such as color copying machine and color printer which is provided with developing units for use with colored toners other than black in addition to a developing unit which is used for black toner.
2. Description of Related Art
In a colored image forming apparatus, an electrostatic latent image which is to be developed by a predetermined color is formed on a photoconductor by optical system of copying machine or printer. Then, the electrostatic latent image is developed by a developing unit which accommodates a predetermined colored toner to be used for development. The developed image after development is transferred onto a transfer material, intermediate transfer belt or the like on the photoconductor. Thereafter, an electrostatic latent image which is to be developed by next predetermined color is formed on the photoconductor by optical system. The electrostatic latent image thus formed is developed by a developing unit which accommodates the next predetermined colored toner to be used for development. The developed image thus obtained is transferred onto the transfer material, intermediate transfer belt or the like on the photoconductor by overlaying it on the image previously transferred. After each transfer process, the developer remained on the photoconductor is cleaned by a cleaning device.
However, when colored toners other than black toner are cleaned, residual developer on the photoconductor is not well cleaned compared with the case when black toner is cleaned, and it causes to readily produce filming on the photoconductor by colored toners other than black toner. The cause is not known yet, however, it may be attributed to dye or pigment which is a component of toners. A resin of low melting viscosity is often used for colored toners other than black toner in order to produce clearer color and also to have satisfactory permeability when image is transferred onto a sheet used for overhead projectors. In this case, however, it tends to further deteriorate cleaning capability.
Japanese Patent Publications TOKKO SHO No. 51-1130, TOKKAI SHO No. 57-204576 and TOKKAI SHO No. 55-89882 disclose ideas to solve these problems.
In the first publication, there is disclosed an idea for improving toner cleaning efficiency by mixing toner with small amount of low adhesive polymeric additive in smaller average particle size than that of the toner of each developer. In the second publication, an idea is disclosed wherein each developer is mixed with an abrasive for removing a matter adhered to the photoconductor when cleaning process is conducted. In the third publication, there is disclosed an idea for removing a matter adhered to the photoconductor with said resin by providing a grinding device aside from a cleaning device.
However, in the colored image forming apparatus, it takes labors to mix proper amount of suitable polymeric additive or abrasive with each developer and it eventually becomes expensive. Moreover, it is not preferable for use in forming a colored image which requires delicate tone since it badly affects the clearness of color and permeability when the additive or abrasive are mixed with colored toner other than black toner.